Process for bonding by molecular adhesion

ABSTRACT

The invention relates to a process for bonding by molecular adhesion of two substrates to one another during which the surfaces of the substrates are placed in close contact and bonding occurs by propagation of a bonding front between the substrates. The invention includes, prior to bonding, a step of modifying the surface state of one or both of the surfaces of the substrates so as to regulate the propagation speed of the bonding front. The surface can be modified by locally or uniformly heating or roughening the surface(s) of the substrate(s).

CROSS-REFERENCE TO REPLATED APPLICATIONS

This application is a division of application Ser. No. 11/357,771 filedFeb. 17, 2006 now U.S. Pat No. 7,601,271.

BACKGROUND

The field of the invention is that of bonding two substrates to oneanother by molecular adhesion. The invention generally concerns aprocess and bonding equipment. It extends likewise to the formation of astructure comprising a thin layer made of a semiconductor material upona support substrate. To form such a structure, the typical procedure isto place a donor substrate in close contact with the support substrate,so as to effect bonding by molecular adhesion of the substrates to oneanother. This is then followed by the transfer of a part of the donorsubstrate to the support substrate to form the thin layer on the supportsubstrate.

Bonding by molecular adhesion (either ‘direct wafer bonding’ or ‘fusionbonding’) is a technique that enables two substrates having perfectlyflat surfaces (e.g., polished mirror surfaces) to adhere to one another,without the application of adhesive (gum type, glue, etc.). The surfacesin question are generally those of substrates made of an insulatingmaterial (for example, quartz or glass) or a semiconductor material (forexample Si, GaAs, SiC, Ge, etc). Bonding is typically initiated by localapplication of light pressure to the two substrates when they are placedin close contact. A bonding front then spreads over the entire extent ofthe substrates in a few seconds.

The bonding energy obtained at ambient temperature is generally lowenough relative to that observed between two solids in covalent, ionicor metallic connection. For numerous applications, however, bonding isreinforced by carrying out thermal annealing. In the case of a siliconsurface molecularly bonded to another silicon or silicon oxide surface,the bonding energy reaches a maximum after a bonding reinforcingannealing carried out at temperatures on the order of 1100° C. to 1200°C.

In addition, to obtain satisfactory bonding of two substrates, thetypical procedure prior to bonding is the preparation of one or both ofthe surfaces to be bonded together. Enhanced bonding is intended toincrease the mechanical performance of the bonded substrates or to boostthe quality of the bonding interface.

An example of such a treatment for increasing the mechanical behaviorbetween the substrates during bonding is the preparation of the surfacesto be bonded with the aim of making them more hydrophilic. Within thescope of hydrophilic bonding, the following properties are preferred forthe surfaces to be bonded.

the absence of particles;

the absence of hydrocarbons;

the absence of metallic contaminants

a low surface roughness, typically less than 5 Å RMS;

strong hydrophily, that is, a substantial density of Si—OH silanol bondsterminating the surfaces to be bonded together.

The preparation of the surfaces to be bonded is generally completed byutilizing one or more chemical treatments. By way of example of chemicaltreatment prior to (hydrophilic) adhesion, the following can bementioned:

-   -   cleaning of RCA type, namely the combination of a SC1 (NH₄OH,        H₂O₂, H₂O) bath adapted to shrinkage of the particles and the        hydrocarbons and a SC2 (HC₁, H₂O₂H₂O) bath adapted to shrinkage        of the metallic contaminants;    -   cleaning with an ozone solution (O₃) adapted to shrinkage of the        organic contaminants;    -   cleaning with a solution containing a mixture of sulfuric acid        and oxygenated water (or SPM solution, Sulfuric Peroxide        Mixture).

The preparation of the surfaces to be bonded can likewise comprisemechanical preparation of the surfaces (light polishing, brushing),either alone or in combination with the chemical treatment.

As a complement to conventional methods of bonding by molecularadhesion, strong bonding techniques at low temperature have beendeveloped more recently to make heterostructures (two materials ofdifferent types) more readily adhere to substrates comprising partiallyor totally manufactured electronic components (aka patterned substrateand structured wafer), or even for adhering substrates that are capableof being altered during annealing at high temperatures. Bonding bymolecular adhesion with plasma activation is an example of such a strongbonding technique that can be carried out at low temperature. Exposureof one or both surfaces to a plasma prior to bonding allows strongbonding energy to be reached after relatively short reinforcingannealing of the structure (around 2 hours) at a relatively lowtemperature (typically less than 600° C.). As a reference for thisteaching, the following articles can be mentioned:

“Effects of plasma activation on hydrophilic bonding of Si and SiO₂” T.Suni et al., J. Electroch. Soc, Vol. 149. No. 6, p. 348 (2002);

“Time-dependent surface properties and wafer bonding of O₂-plasmatreated silicon (100) surfaces”, M. Wiegand et al., J. Electroch. Soc.Vol. 147, No. 7, p. 2734 (2000).

It is evident that the different techniques for surface preparationmentioned earlier systematically incorporate at least one humid stage,that is, at least rinsing of the surfaces by deionized water. Thesubstrates are then dried, for example by centrifuging (dry spin). As afunction of their degree of hydrophily, the surfaces of the substrateshave, after drying, several monolayers of adsorbed water, with thesemonolayers being at the origin of the intermolecular forces responsiblefor adhesion during contacting of the surfaces.

Also, bonding by molecular adhesion of substrates to one anothergenerally causes defects. Notable examples of such defects includedefects of the bubble type (or bubbles) at the level of the bondinginterface between the two substrates, as well as defects of edge type(or edge voids) at the level of the thin layer of the final structure(the thin layer on support substrate that is obtained after transfer).Bubbles are understood to be defects that result from gas and/or watercombining at the bonding interface between the two substrates. Bubblescan appear after the application of a low budget thermal to the adheredstructure (for example after application of thermal annealing at 200° C.over 2 hours) and are observable by inspection of the bonding interfaceusing an infrared camera, or by acoustic microscopy. The bubbles will beresponsible for the presence of non-transferred zones at the level ofthe final structure obtained after transfer. The article“Low-Temperature Wafer Bonding, Optimal O₂ Plasma Surface PretreatmentTime”, by X. Zhang and J-P. Raskin in Electrochemical and Solid-StateLetters, 7 (8) G 172-G174 (2004), illustrates the phenomenon of theformation of bubbles at the bonding interface. Edge voids are understoodto mean defects which result from bonding and which are typicallyobserved at the periphery of the final structure (generally in the formof a circular plate).

An application of direct bonding is that carried out within the scope ofproducing structures of the Semiconductor On Insulator or SeOI type, andin particular for Silicon On Insulator or SOI structures. Includedwithin this scope of this invention are substrates to be bonded where atleast one has a surface layer of oxide; by way of example, Si/SiO₂bonding or SiO₂/SiO₂ bonding which are typically undertaken to form aSOI structure.

There are three main methods for producing SeOI structures by directbonding: to well known SMART-CUT® process, or the so-called BSOI (andBESOI) or ELTRAN® processes. A description of the processes associatedwith each of these methods can be found in the text entitled ‘Siliconwafer bonding technology for VLSI and MEMS applications’, by S. S. Lyerand A. J. Auberton-Hervé, IEEE (2002). Despite the accepted use of theseprocesses, defects of the edge voids type, cause by the bonding step,can often appear after transfer of the thin layer from the donorsubstrate to the support substrate.

As is schematically illustrated in FIG. 1 in terms of forming a SOIstructure, an edge void P is a hole (of a diameter typically between 100μm and 1 mm) in the thin transferred layer which corresponds to a zoneof the donor substrate not transferred to the support substrate A. Theedge voids appear most often at the edge (peripheral zone) of the “thinlayer on support substrate” structure (circular wafer); and they areusually located at a distance of typically between 1 mm and 5 mm of thewafer edge.

The edge voids are macroscopic defects connected to poor bonding at theedge of the wafers. They can be killer defects because, in the absenceof a thin layer acting as an active layer for the formation ofelectronic components at the location of an edge void, no component canbe manufactured at this location. Given the size of the edge voids, anelectronic component comprising at least one edge void is necessarilydefective.

In addition, a transfer process of the SMART-CUT® type is notablyinteresting in that it allows for recycling of the donor substrate. Sowhen adhesion of a recycled donor substrate is completed (that is, adonor substrate already having served for removal and transfer of a thinlayer; known as ‘refresh’ wafer), a greater number of edge voids isobserved than when bonding of an original donor substrate is completed(i.e., one having never served to remove and/or transfer of a thinlayer; known as ‘fresh’ wafer). This increased presence of edge voidstends to prohibit the recycling of such wafers, thus defeating one ofthe main reasons for using the SMART-CUT® process.

Since the presence of edge voids induces losses in terms of quality andyield, there is thus a need to prevent the formation of such defects. Ithas been proposed in European patent application EP 1 566 830 to limitthe number of defects of void type at the edge of a SOI wafer obtainedas a result of molecular bonding. According to this application, thesedefects are always located at a specific position relative to the centerof the wafer, and seem to be due to the configuration of the edges ofwafers. Therefore, to decrease the number of defects, this applicationproposes modifying the configuration of wafer edges during manufacture.More precisely, this application proposes modifying the curve of theedge drops, in regions ranging from 3 mm to 10 mm from the periphery ofthe wafer. This solution thus has the disadvantage of requiring previousmechanical intervention on the wafers.

Another application of direct bonding is that of Si/Si bonding of DSBtype (Direct Si Bonding). As mentioned earlier, defects of bubble typeare all the same capable of appearing at the bonding interface. Onesolution for reducing the formation of bubbles in this process consistsof producing plasma activation of the surfaces to be bonded, so as toobtain good adhesion energy, but this solution has not been found to beentirely satisfactory for reducing the number of bubbles at the bondinginterface.

Thus, improvements in these type processes are desired and necessary.

SUMMARY OF THE INVENTION

The present invention provides bonding techniques that rectify thedisadvantages of the state of the art. The invention generally relatesto a method for bonding two substrates together by molecular adhesion oftheir surfaces which comprises preparing the surfaces of the substrateswith a flatness sufficient to facilitate bonding by propagation of abonding front when the surfaces of the substrates are placed in contactwith each other, modifying the surface of one or both of the substratessufficiently to regulate propagation speed of the bonding front toreduce bubble or voids between the substrates after bonding. Enhancedbonding can be achieved by modifying the surface(s) to decrease wateradsorbed thereon. Thereafter, the surfaces of the substrates can beplaced in contact together to effect bonding with the modifiedsurface(s) providing a sufficiently reduced speed of the bonding from toreduce the number of bubbles or void defects therebetween compared tobonding with non-modified substrate surfaces.

Various ways can be used to modify the surface of one or both of thesubstrates, including: heating all or a portion of the surface(s),preferably by conduction from a heated plate or from infrared lamps orby forming a rough layer on the surface(s) according to differenttechniques. It is also possible to conduct plasma activation after thesurface(s) modifying step to further enhance bonding. These variousprocedures are defined in the following detail description.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other features and advantages of the present invention will emerge moreclearly from reading the following detailed description of preferredembodiments, given by way of non-limiting examples, and done inreference to the attached drawings, in which:

FIG. 1 is a schematic illustration of a method of forming a SOIstructure as known in the art;

FIG. 2 illustrates the formation of edge voids as a function of thelocation of the point of initialization of adhesion;

FIG. 3 represents different diagrams of equipment according to thesecond aspect of the invention;

FIGS. 4 and 5 respectively show uniform heating and localized heating oftwo substrates to be placed in close contact to bring about theirbonding by molecular adhesion;

FIG. 6 is a diagram illustrating the formation of edge voids as afunction of the propagation speed of the bonding front;

FIG. 7 is a diagram showing an embodiment of the bonding processaccording to the first aspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, the invention concerns a bonding process by molecularadhesion of two substrates to one another, during which the surfaces ofthe substrates are placed in close contact and bonding occurs bypropagation of a bonding front between the substrates, which comprises,prior to bonding, a step consisting of modifying the surface state ofone or both of the substrates so as to regulate the propagation speed ofthe bonding front.

Certain preferred, though non-limiting, aspects of this embodiment arethe following:

-   -   modification of the surface state is a decrease in the thickness        of a layer of water adsorbed on the surface of one or both of        the substrates to be bonded;    -   modification of the surface state is completed by heating;    -   heating is applied before the substrates are placed in close        contact with one another and at least until initiation of        bonding;    -   heating is effected over a period of between 1 and 90 seconds,        preferably for 30 seconds;    -   heating is effected by thermal conduction via transmission of        heat from a plate supporting one of the substrates to be bonded,        with the same or preferably separate plates used for heating        each substrate;    -   heating is effected by radiation from a lamp lighting one of the        substrates to be bonded;    -   the lamp is a lamp radiating in the infrared range, preferably        with a wavelength of between 0.8 μm and 5 μm;    -   heating is effected at a temperature of between about 30° C. and        90° C.; preferably between about 50° C. and 60° C.;    -   heating can applied uniformly over the entire surface of the        substrate to be bonded; or    -   heating is applied locally at a peripheral zone of the substrate        to be bonded;    -   adhesion is initiated in the center of the substrates, with the        heated peripheral zone covering the entire circumference of the        substrate to be bonded;    -   adhesion is initiated at the edge, with the heated peripheral        zone defining an arc of a circle diametrically opposed to the        edge of initiation and limited by an angle at the center of        around 120°;    -   before bonding and following the modification stage of the        surface state, the process comprises a stage of plasma        activation of one or both of the surfaces to be bonded.

Rather than heating, additional techniques can be used to modify thesurface(s) of the substrate to be bonded. Certain aspects, againpreferred though non-limiting, of this second embodiment of this processare the following:

-   -   modification of the surface state is effected by roughing the        surface:    -   the modification step of the surface state includes forming a        rough surface layer on one or both of the substrates to be        bonded;    -   to form the rough layer, the process comprises a thermal        oxidation operation of one or both of the substrates so as to        form a thermal oxide layer thereon and an operation for treating        the thermal oxide layer adapted to etch the oxide layer:    -   treatment of the thermal oxide layer can be accomplished by        other chemical treatments rather than etching;    -   the layer of thermal oxide is a layer of SiO₂ and the chemical        treatment is SC1 treatment conducted at a temperature of between        50° C. and 80° C. over a period of longer than three minutes,        preferably over 10 minutes;    -   to form the rough layer, the process comprises an operation for        depositing a surface layer of oxide on the surface(s) of one or        both of the substrates;    -   the surface layer to be deposited is a layer of TEOS oxide, a        layer of LTO oxide, or a layer of nitride;    -   for further enhancement of bonding, the process includes, before        bonding and after the modification step of the surface(s), a        plasma activation stage of one or both of the surfaces to be        bonded.

Another aspect of the invention relates to equipment for bonding bymolecular adhesion the two substrates to one another. Such equipmenttypically includes means for modifying, prior to bonding, the surface(s)of one or both of the substrates so as to regulate the propagation speedof the bonding front. In particular, heating means can be used to raiseand control the temperature of one or both of the substrates beforebonding.

The invention also concerns, under another aspect, a process forformation of a structure comprising a thin layer made of a semiconductormaterial on a support substrate, comprising the steps of placing inclose contact a donor substrate, for example a ‘refresh’ substrateoriginating from recycling, with the support substrate so as to carryout bonding by molecular adhesion of the substrates to one anotherfollowing the propagation of a bonding front between the substrates, andtransfer of part of the donor substrate to the support substrate so asto form the thin layer on the support substrate, characterized in thatit comprises prior to bonding a step consisting of modifying the surfacestate of the donor substrate and/or of the support substrate so as toregulate the propagation speed of the bonding front. And the inventionextends of course to the “thin layer on support substrate” structuresobtained from implementing this process.

According to the first aspect, the invention concerns a process ofbonding by molecular adhesion of two substrates to one another, duringwhich the substrates are placed in close contact and bonding occurs bypropagation of a bonding front between the substrates. It is understoodthat the invention is not limited to bonding by molecular adhesion oftwo substrates to one another, but extends likewise to the formation ofa structure comprising a thin layer of semi-conductor material on asupport substrate, during which bonding by molecular adhesion is carriedout of a donor substrate with the support substrate, followed bytransfer of the thin layer from the donor substrate to the supportsubstrate.

As mentioned earlier, the SMART-CUT®, BSOI (and BESOI), and ELTRAN®processes are examples of processes that use bonding by molecularadhesion for transferring useful layers. According to the SMART CUT®process, prior to bonding an embrittlement zone (or zone of weakness) isformed by implantation of atomic or ionic species in the thickness ofthe donor substrate, and after bonding detachment of the donor substrateis made at the embrittlement zone to transfer the thin layer to thesupport substrate. To carry out molecular adhesion, typical procedure isto place the donor substrate and the support substrate in close contact,then to initiate bonding by local application of light pressure on thetwo substrates placed in close contact. A bonding front then spreadsover the entire extent of the substrates.

The origin of the defects resulting from bonding is not determined withcertainty these days. Analysis shows that within the scope of Si/Si DSBtype bonding, the bubble-type defects appear more particularly since nooxide layer (other than native oxide) is present at the surface of thesubstrates to be bonded. The applicant estimates in effect that in thecase of adhesion with the presence of a layer of oxide (for example forSi/SiO₂ bonding, such as in view of formation of a SeOI structure), thewater and gas encapsulated during bonding or appearing later (forexample during thermal consolidation treatment of the bonding)preferably diffuse in the oxide layer and are thus less inclined todiffuse to the bonding interface. However, in the case of DSB bonding,in the absence of such a layer of oxide, it is believed that the waterand the gas encapsulated during bonding or appeared consequently have atendency to diffuse to the bonding interface, where they will combine toform bubbles.

With respect to the edge void-type defects, analysis shows that theseare defects that form with closing of the space between the surfacesduring bonding, and in particular where the bonding front meets the edgeof the bonded substrates. The illustrations of FIG. 2 show analysisresults concerning the formation of edge voids. In FIG. 2, the arrowsrepresent the direction of propagation of the bonding front, the pointedlines represent the position of the bonding front at different instants,and the points represent the edge voids. As is illustrated to the leftin FIG. 2, it has been determined that when bonding is initiated at thecenter of the surfaces (by local application of pressure, for example),edge voids are potentially found over the entire periphery of thesupport substrate. However, as illustrated on the right of FIG. 2, whenbonding is initiated on board the wafer (typically at the level of arecess known as a ‘notch’ made on board the wafer to facilitate itsmanipulation), edge voids can appear in a peripheral zone of the supportsubstrate describing an arc of a circle diametrically opposed to thepoint of initiation, limited by an angle at the center of around 120°.

In general, the present invention limits or even prevents the formationof defects caused by bonding by regulating the propagation speed of thebonding front. ‘Regulate’ is understood to mean to control, maintain andconserve the command of evolution of a phenomenon. Within the scope ofthe present invention, the phenomenon is that of propagation of thebonding front, and evolution of the phenomenon corresponds to thepropagation speed of the bonding front. The speed of propagation is moreprecisely regulated so as to be reduced relative to the speed usuallyobserved in the absence of such control. To allow regulation of thepropagation speed of the bonding front, the present process includescarrying out, prior to bonding, a modification step of the surface ofone or both of the surfaces to be bonded.

This modification step is conducted after the surfaces of the substratesare flattened and prepared for molecular bonding immediately prior tocontacting the surfaces together to initiate bonding. The modificationstep is primarily intended to remove water or water layers from one orboth of the surfaces to facilitate bonding, but it does notsignificantly detract from the surface preparation steps or steps. Whilethe modification step also slightly reduces the speed of propagation ofthe bonding front, it does not prevent such front from proceeding orotherwise inhibit molecular bonding. It was unexpected to find that sucha modification step would reduce bubbles or voids while not hinderingthe molecular bonding of the substrates, since it is known in the art toprepare such surfaces to be as smooth as possible to facilitate thebonding step. Rather than preserving such smooth surfaces for bonding,the application of heat or minor roughening of the substrate surfacesimmediately prior or directly before bonding according to the presentinvention now provides a measurable reduction or elimination of bubbleor edge void defects.

According to a first embodiment, this means in particular to control thequantity of water adsorbed at the surface, and more precisely todecrease, without completely eliminating all the same, the thickness ofthe layer of water adsorbed relative to the thickness of the layer ofwater normally adsorbed. In other terms, this means to reduce the numberof monolayers of water adsorbed at the surface. Bonding by molecularadhesion is also known as direct bonding, because it concerns bondingnot requiring the application of an adhesive (of gum or other gluetype). It is really the water adsorbed on each of the surfaces (severalmonolayers of water) and placed in contact which acts as adhesive andensures adhesion by means of Van der Waals force. For this reason, itwas previously considered that several monolayers of water must bepresent on the surface in order to obtain good molecular bonding.

Within the scope of this first embodiment, the surface state of asubstrate is modified, and the propagation speed of the bonding front isthus regulated, by playing with the temperature of the substratesurface(s) prior to bonding. Bonding by molecular adhesion isclassically carried out at ambient temperature (20° C. to 25° C.),whether in the case of manual bonding or in the case of automaticbonding. In contrast, it has been found that edge voids and bubbles canbe partially or totally eliminated when the substrates are pre-heatedbefore and up until being placed in close contact. The heating actuallycauses modification of the surface state of the substrates placed incontact, enabling the propagation speed of the bonding front to bereduced. By controlling this heating, it is possible to regulate thepropagation speed of the bonding front, that is, to control thereduction in the propagation speed of the bonding front. And while thisstep removes some of the layers of water from the surfaces to be bonded,it allows sufficient water to remain so that molecular bonding canoccur, with only a slight reduction in the propagation speed on thebonding front but a significant reduction of defects in the bondedsubstrate assembly.

FIG. 6 illustrates a curve showing the number of edge voids Np observedafter transfer as a function of the propagation speed of the bondingfront Vp (expressed in centimeters per second) for Si on SiO₂ bonding.FIG. 6 is only given by way of illustration. In addition, the differentnumerical examples obviously depend on the substrates utilized foradhesion (substrate originating from recycling ‘refresh substrate’, ornot, ‘fresh substrate’; type of material comprising the substrate, inparticular its flexibility, etc.).

For low bonding speeds (typically less than 1.7 cm/s), no edge voidshave been encountered. For a speed of 1.7 cm/s, between 0 and 1 edgevoids are observed. The number of edge voids then increases rapidly whenthe propagation speed of the bonding front rises. Thus, 5 edge voids canbe counted at a speed on the order of 2 cm/s and between 50 and 100 edgevoids for a speed of the order of 3 cm/s.

When standard cleaning only of the substrates to be bonded is carriedout prior to bonding (for example RCA), the overall propagation speed ofthe bonding front is found to be between 1 cm/s and 2.5 cm/s (G_(N)range in FIG. 6). On average, there are no edge voids observed on allstructures obtained after transfer when bonding is completed followingRCA cleaning. But certain structures have a significant number of edgevoids; these typically are those for which the bonding front haspropagated at a speed greater than 1.7 cm/s.

It should be mentioned here that good bonding energy and rapidpropagation of the bonding front are generally associated. U.S. Pat. No.6,881,596 proposes to determine the quality of the bonding interface bymeasuring the propagation speed of the bonding front. The article“Dynamics of a Bonding Front” by Rieutord, Bataillou and Moriceau inPhysical Review Letters, PRL 94, 236101 proposes a formula (see equation5) indicating that the propagation speed increases if the bonding energyaugments. And as already mentioned above, plasma activation can havebeen carried out as a complement to standard RCA cleaning. This plasmaactivation focuses especially on increasing the bonding energy.

It has been found that the propagation speed is more significant afterplasma activation of the substrates to be bonded. FIG. 6 schematicallyillustrates the propagation speed within the scope of RCA cleaningtreatment and plasma activation (here activation of only one of thesurfaces to be bonded). As can be noted, the increase in propagationspeed is accompanied by the formation of a significant number of edgevoids (see G_(N+P) range in the figure).

With reference to the description of the invention, prior to bonding thesurface state of one or both of the substrates to be bonded is modifiedso as to regulate the propagation speed of the bonding front. Within thescope of the example in FIG. 6, this regulation is provided so that thespeed of the bonding front is in the G_(R) range (typically above 0.6cm/s and below 2 cm/s and preferably between 0.8 cm/s and 1.7 cm/s; oran overall bonding time of between 18 and 35 seconds for substrates inthe form of wafers of 300 mm in diameter) so as to prevent the formationof edge voids.

A first application to be made of regulation by heating concerns theformation of a SeOI structure following bonding of two substrates,whereof one at least has a surface oxide layer. In fact, by reducing thespeed of the bonding front, better-quality bonding (in particular on theedge of the wafer) can be obtained, thus preventing the non-transfer ofcertain zones of the donor substrate to the support substrate andconsequently the formation of edge voids. It has been found that byusing such hot contact, no edge voids are observed, whereas as many as80 to 100 edge voids can be counted on SOI structures originating frombonding by being placed in contact at ambient temperature. It will benoted that obtaining this good-quality bonding enables recycling, thatis, the utilization of substrates of the refresh type.

Within the scope of this first application, heating is conducted at atemperature of between 30° C. and 90° C., preferably between 50° C. and60° C. This temperature range results from a compromise between theappearance of edge voids at a temperature close to 25° C. (ambienttemperature) and observation of a drop in bonding energy at excessivetemperature. If the substrates are heated to an excessive temperature, amajority of the water adsorbed at the surface (even all of it) willevaporate, and the bonding force risks dropping sharply. Other types ofdefects are capable of being caused by excessively low bonding energy.At the extreme, molecular adhesion may not even be produced.

To illustrate the drop in bonding energy, reference can be made to thearticle by Suni et al. in J. Electrochem. Soc. Vol. 149 No. 6 pp. 348 to351, 2002, entitled “Effects of Plasma Activation on Hydrophilic Bondingof Si and SiO₂”, in which it is mentioned that the bonding energy dropsfrom 2.5 J/m² (case of bonding at ambient temperature) to 1 J/m² (caseof bonding conducted at 150° C.), all this after bonding reinforcingannealing carried out at 200° C. Within the scope of this firstapplication, the invention proposes to use the lowest of thetemperatures allowing the edge voids to disappear, so as not to cause aharmful drop in bonding energy.

With reference to the specific example in FIG. 6, the aim is apropagation speed of the bonding front of not less than 0.8 cm/s so asnot to cause a harmful drop in the bonding energy, and in any case lessthan 1.7 cm/s to avoid the formation of edge voids. Therefore,irrespective of the surface preparations made prior to bonding, the aimhere is a speed in the G_(R) range, especially by reducing this speed byheating, in a controlled manner. Mention is made that plasma activationcan likewise be utilized within the scope of regulating the speed of thebonding front, with plasma activation effectively causing accelerationof the propagation of the bonding front (or an increase in thepropagation speed).

A first variant of this first possible embodiment consists of uniformlyheating the whole of one or both of the substrates to be bonded. Asecond variant comprises conducting localized heating of one or both ofthe substrates to be bonded, limited to the zone where the edge voidsmight disappear. As discussed hereinabove with respect to FIG. 2, thisis about the termination zone of the bonding front, localization and theextent of this zone depending on the manner in which bonding has beeninitiated.

Accordingly, when the bonding of two circular substrates is initiated atthe center, the invention proposes heating the entire peripheral zone(that is, all of the circumference of the substrate). By way of purelyillustrative example, this peripheral zone can be considered asoccupying a peripheral band of 50 mm wide from the edge of a wafer 300mm in diameter.

However, when the bonding of two circular substrates is initiated at theedge, the invention advantageously proposes to heat only the edgediametrically opposite to this point (in particular the peripheral zonedelimited by an angle at the center of around 120°). By employinglocalized heating, the bonding front is uniquely slowed locally. Thisprevents the formation of edge voids without the rest of the bonding(non-heated zone) being altered and undergoing a loss in terms ofbonding energy.

The heating (whether localized or extended to all of one or both of thesubstrates) can be carried out by thermal conduction. There can beprovision for the plate on which repose one of the substrates to bebonded (“chuck” plate) to transmit its heat. It can also be providedthat heating is carried out by radiation, for example by using one ormore halogen lamps illuminating all or part of the substrates to beheated.

Heating is completed before the surfaces to be bonded (the substratesthen typically being placed opposite, separated by a few millimeters bymeans of spacers) are placed in close contact and at least untilinitiation of bonding (the substrates then having been placed in closecontact). In particular, heating is carried out such that the zone wherethe edge voids are capable of appearing remains at the desiredtemperature until the substrates are adhered in this zone (byhypothesis, the water desorbed locally does not have to be able tocondenser before the bonding is finished).

The duration of heating depends largely on the device utilized to raisethen control the temperature of the heated zone. It is typically between1 and 90 seconds. By way of example, with a 500 W halogen lamp, theduration of heating is typically between 30 and 90 seconds. It is notedthat this time range likewise depends on other parameters, such as forexample the distance between the lamp and the substrates.

The spectral distribution of the lamp utilized to heat and have thewater desorbed is likewise a parameter influencing the duration ofheating. In fact, a lamp mainly emitting light in the infrared mode(length of average wave around 3 μm, typically comprises between 0.8 μmand 5 μm) is particularly efficacious to effect desorption of water (theabsorption band of the molecules of water are effectively close to 3 μm)and thus leads to ultrarapid heating, quasi-instantaneously decreasingthe thickness of the layer of water adsorbed. In addition, theutilization of such infrared radiation helps to selectively heat theadsorbed water while heating the silicon wafer much less, a relativelytransparent material in the infrared.

A second application from heating relates to carrying out adhesion ofDSB type. Bubbles can be partially or totally eliminated within thescope of Si/Si adhesion of DSB type when the substrates are heateduniformly before and until they are placed in close contact. IRobservations have thus shown a significant reduction, even that thebubbles have disappeared. This is believed to be due to the fact thatheating in effect diminishes the thickness of the layer of wateradsorbed at the surface of the substrates. The quantity of water (and/orgas) capable of diffusing at the bonding interface is now reduced, andthis allows the appearance of bubbles at the bonding interface to bereduced or to completely disappear.

The previous discussion concerning the different ways of carrying outheating likewise applies to this second application, by noting all thesame that uniform heating of one or both of the substrates to be bondedis preferably carried out here. DSB bonding can especially be utilizedto create Si/Si bonding of substrates having different crystallineorientations, or again substrates having different doping, or evensubstrates having different levels of constraints. Following transfer ofpart of one of the substrates to the other, to form a thin layer there,‘thin layer on support substrate’ structures are produced, for which thethin layer on one side and the support substrate on the other havedifferent properties.

In order to enable regulation of the propagation speed of the bondingfront, the invention proposes to carry out, prior to adhesion, amodification step of the surface state of one or both of the surfaces tobe bonded. According to a second possible embodiment, the surface stateof a substrate is modified by altering the surface roughness prior tobonding. This second embodiment is more particularly adapted to theformation of a SeOI structure for which an insulating layer isinterposed between the thin layer and the support substrate (also knownas a buried layer). This insulating layer is normally formed by thermaloxidation of the donor substrate and/or of the support substrate oragain by depositing a layer of oxide on the surface of the donorsubstrate and/or of the support substrate.

This embodiment proves especially advantageous for the formation of aSeOI structure having an ultra thin insulating layer. In fact, it provesto be particularly difficult using techniques of the prior art to effectadhesion and/or transfer without defect with such a ultra thin layerinterposed in between the thin layer and the support substrate. By ultrathin insulating layer, it is generally understood here a layer whichthickness is less than 500 Å, even less than 200 Å. The propagationspeed of the bonding front is sensitive to the surface state of thesubstrates put in contact. The different surface cleaning and/ortreatment options made prior to bonding, but also the surface roughness,thus influence the rapidity with which the bonding front spreads. Withinthe scope of this second embodiment the surface roughness of a layer ofoxide is controlled so as to regulate the propagation speed of thebonding front, that is, in such a way as to control the reduction inpropagation speed of the bonding front. Since the bonding front isslowed, the result from this is a drop in the number of edge voids atthe edge of the wafer.

A first variant to this second embodiment consists, prior to bonding, ofmodifying the surface state of a thermal oxide layer formed on thesurface of one of the donor or support substrates and intended to formthe buried layer by conducting ‘aggressive’ cleaning of the surface ofthe layer of oxide. This cleaning is to be carried out prior to anyplasma activation. Of course, a thermal layer of oxide can be formed oneach of the donor and support substrates and such ‘aggressive’ cleaningcan be performed on one or both of the surfaces of the thermal oxidelayers.

For example, in the case of formation of a SOI structure comprising aburied, ultra thin thermal oxide layer of the order of 250 Å to 500 Å(such a layer is known as ‘Ultra Thin BOX’), adapted chemical treatmentcan be carried out so as to lightly etch the surface of the oxide layer.For example, SC1 treatment is applied according to conditions(temperature, duration) more significant than those respected duringstandard cleaning treatment. SC1 treatment can thus be applied withinthe scope of the invention at a temperature of between 50° C. and 80°C., for example of the order of 70° C., with a treatment durationgreater than 3 minutes, for example of the order of 10 minutes.

FIG. 7 is a diagram illustrating the bonding process according to thisvariant to the first aspect of the invention. At stage 1, there are twosubstrates A and B. At stage B, procedure involves thermal oxidation ofthe substrate A to form a layer of oxide O at the surface of thesubstrate A. At stage 3, aggressive cleaning of the layer of oxide O iscompleted to obtain a rough layer O′ at the surface of the substrate A.At stage 4, the substrates A and B are placed in close contact by way ofthe rough layer O′, and bonding is initiated such that a bonding frontspreads to the bonding interface.

A second variant of this second embodiment consists, prior to bonding,of modifying the surface state of one or both of the donor and supportsubstrates by depositing a rough layer on one or both of the substrates.Within the scope of the example of FIG. 7, it is understood thataccording to this second variant steps 2 and 3 of FIG. 3 are completedat the same time by deposit of a rough layer O′ on surface of thesubstrate A. It is required for example to deposit a TEOS layer of oxide(for example deposited by LPCVD—Low Pressure Chemical VaporDeposition—or by PECVD—Plasma Enhanced Chemical Vapor Deposition), of aLTO layer of oxide (by chemical reaction of silane with the oxygen), oragain a layer of nitride. Depositing is effected according to adapteddepositing conditions, as a function of the desired final thickness, totarget a certain roughness, and in particular a roughness such that thesurface state limits the propagation speed of the bonding front.

By way of example, a roughness of a TEOS layer of oxide of between 2 ÅRMS and 5 Å RMS limits the propagation speed of the bonding front, atthe same time as retaining good bonding energy. It is specified herethat the TEOS deposit is particularly adapted for the formation of ultrathin oxides (thickness less than 500 Å, or even less than 250 Å). Ineffect, the roughness of such deposited oxide is typically what iswanted, without requiring additional treatment. Depositing conditionshelping to target adequate roughness are for example the following:pressure of between 300 mT and 700 mT; temperature of between 650° C.and 700° C. It is noted that the increase in pressure, as also theincrease in temperature, results in a decrease in roughness. It is notedthat this second variant is applicable both to thick deposits of oxidesand for deposits of ultra thin films which subsequently undergo or notplasma activation treatment.

The invention also proposes, according to a second aspect, equipmentenabling molecular adhesion of two substrates to one another. Equipmentfor manual bonding by molecular adhesion usually comprises a support onwhich is placed a first substrate, the second substrate then beingreturned with respect to the first substrate. The initiation of bonding(local pressure) is assured manually by means of a stylus.

Automated equipment may comprise especially:

-   -   an aligner for registering the centre and orientation of the        wafer (in particular thanks to the presence of a recess known as        ‘notch’);    -   one or more station(s) for surface preparation prior to bonding        (operations for cleaning, rinsing, drying, etc.);    -   an adhesion support receiving the first substrate, then the        second at the end of bonding. Spacers can likewise be provided        to maintain the second substrate a few millimeters above the        first before they are placed in contact;    -   an automatic piston ensuring initiation of bonding;    -   one or more loading ports receiving the cassettes of substrates        to be bonded or already bonded;    -   a robot ensuring transport of the wafers from one element of the        equipment to another.

In general, the equipment according to the second aspect of theinvention reprises the classical configuration of bonding equipment bymolecular adhesion (manual or automated) but further comprises means formodifying, prior to adhesion, the surface state of one or both of thesubstrates to be bonded. These are especially heating means for raisingand controlling the temperature of one or both of the donor and supportsubstrates before they are placed in close contact and also during beingplaced in close contact (that is, also during propagation of the bondingfront). These heating means can dispense heat above and/or below thesubstrates ready to be bonded.

By way of example of devices for heating below, and in reference to FIG.3, a ‘chuck’ C can be provided, forming a ‘heating wafer’. This is forexample a chuck in the mass whereof one or more electrical resistors Rare integrated (see illustration to the left in FIG. 3), or a ‘chuck’ Cin the mass whereof means F are integrated for circulating fluid whereofthe temperature is regulated (at the centre in FIG. 3). It is likewisepossible to utilize (to the right in FIG. 3) one or more heating lamps Lthat communicate their heat to the ‘chuck’ or directly illuminate therear face of one of the substrates through a transparent plate (at leasttransparent to IR radiation).

It is evident that in FIG. 3 (as well as in FIGS. 4 and 5, to bementioned hereinbelow) the donor and support substrates are shown asbeing opposite (typically separated by a few millimeters by spacers E),before being placed in close contact. These different techniques can beutilized separately or in combination, locally or on the whole of thechuck. It is possible to produce temperature gradients over the extentof the surfaces, or again to create local control of the temperature.

By way of example of a device enabling heating from above, anarrangement of lamps radiating directly onto on the substrates can beprovided, uniformly or deliberately localized (especially where the edgevoids are capable of appearing). So as to control the temperature of theheated zone (whether part or all of a substrate or the substrates) theequipment can also advantageously comprise a device for measuring thetemperature in the heated zone (not shown; for example in the form of apyrometer or a thermocouple). Equipment can also be available in amanual or automatic version.

Hereinbelow two examples are provided for using the first embodiment ofthe process according to the first aspect of the invention for theformation of SOI structures, within the scope of the transfer process ofSMART CUT® type. A first example is that of uniform heating of thesubstrates by means of a heating chuck. The different stages are thefollowing:

-   -   preparation of the surfaces to be bonded according to humid        cleaning combining RCA cleaning and treatment based on ozone;    -   optional plasma activation (O₂) of the surface of the donor        substrate;    -   cleaning of the surfaces immediately prior to bonding, by        brushing then rinsing with ultra-pure water and drying by        centrifuging;    -   uniform heating of the substrates to be bonded by means of a        heating chuck on which are placed the substrates not yet in        contact but placed opposite one another and separated by a few        millimeters (see FIG. 4). Heating is carried out for several        seconds, according to the power of the heating device (from 1 to        90 seconds, typically 30 seconds),    -   placing the substrates in close contact and initiation of        bonding;    -   termination of heating.

A second example is that of localized heating at the zone where edgevoids are capable of appearing, and comprises the following stages:

-   -   preparation of the surfaces to be bonded according to humid        cleaning combining RCA cleaning and treatment based on ozone;    -   optional plasma activation (O₂) of the surface of the donor        substrate;    -   cleaning of the surfaces immediately prior to adhesion, by        brushing then rinsing with ultra-pure water and drying by        centrifuging;    -   heating of the substrates to be bonded only in the zone opposite        the point of initiation using a lamp L placed above the        substrates not yet placed in contact but opposite one another        and separated by a few millimeters (cf. FIG. 5). Heating is        carried out for several seconds, according to the power of the        heating device (from 1 to 90 seconds, typically 30 seconds);    -   placing in contact and initiation of adhesion (I_(C) localized        initiation at the wafer edge); and    -   termination of heating the wafers.

It is understood that the present invention should not be limited to thepreferred embodiments previously described and that the appended claimsare intended to cover the present disclosure as well as allmodifications that are within the ability of a skilled artisan.

1. A method for bonding two substrates together by molecular adhesion oftheir surfaces which comprises: preparing the surfaces of the substrateswith a flatness sufficient to facilitate bonding by propagation of abonding front when the surfaces of the substrates are placed in contactwith each other; and modifying the surface of one or both of thesubstrates sufficiently to locally regulate propagation speed of thebonding front to reduce bubble or voids between the substrates afterbonding, wherein the modifying of the surface(s) includes roughening oractivating of one or both of the first surfaces.
 2. The method of claim1, wherein the modifying includes roughening the surface of thesubstrate to be bonded by depositing a layer of oxide or nitridethereon.
 3. The method of claim 2, wherein the deposited layer of oxideis a layer of TEOS oxide, a layer of LTO oxide, or a layer of a nitride.4. The method of claim 1, wherein the modifying includes activating ofone or both of the first surfaces of the substrates.
 5. The method ofclaim 4, wherein the activating comprises plasma activation of one orboth of the surfaces to be bonded.
 6. The method of claim 1, whichfurther comprises bonding the substrates together after modifying thesurface(s) of the substrate(s) with the bonded substrates exhibitingreduced bubble or voids therebetween.
 7. The method of claim 6, whereinthe modifying is conducted to provide the bonding front with a speed ofabove 0.6 and below 2 cm/s.
 8. The method of claim 6, wherein thesubstrates are in the form of wafers of 300 mm in diameter, and theroughening is conducted such that bonding is effected in an overallbonding time of between 18 and 35 seconds to avoid forming edge voids.9. The method of claim 6, wherein the bonding is initially conductedlocally on one side of the substrate(s) and the heating is appliedlocally on the substrate(s) on a side that is 180° opposite to the sidewhere bonding is initiated.
 10. The method of claim 6, wherein thebonding is initiated about the center of the substrate(s) and theheating is applied to the entire periphery of the substrate(s).
 11. Themethod of claim 1, wherein the modifying includes roughening thesurface(s) of the substrate(s) to be bonded.
 12. The method of claim 11,wherein the roughening comprises forming a thermal oxide layer on thesurface of the substrate by thermal oxidation followed by etching orother chemical treatment of the oxide layer.
 13. The method of claim 12,wherein the thermal oxide layer is a layer of SiO₂, and the chemicaltreatment is an SC1 treatment performed at a temperature of between 50°C. and 80° C. for a duration of longer than three to ten minutes.
 14. Amethod for bonding two substrates together by molecular adhesion oftheir surfaces comprising: supporting a first substrate and placing asecond substrate upon the first substrate, each substrate having a firstsurface that has a flatness sufficient to facilitate bonding bypropagation of a bonding front when the first surfaces of the substratesare placed in contact with each other, and a second surface that isopposite to the first surface; initiating bonding of the substrates byapplying a force upon the surface of the second substrate; modifying,prior to adhesion, the surface state of one or both of the substrates tobe bonded by applying heating directly to one or both substrates toregulate propagation speed of the bonding front to reduce bubble orvoids between the substrates after bonding.
 15. The method of claim 14,wherein the heating is applied directly to the second surface of thefirst substrate prior to contact with the second substrate.
 16. Themethod of claim 14, wherein the heating is carried out for between 1 and90 seconds at a temperature of between about 30° C. and 90° C. and iscontinued until the first surfaces of the first and second substratesare placed in contact.
 17. The method of claim 14, wherein the bondingis initially conducted locally on one side of the substrate(s) and theheating is applied locally on the substrate(s) on a side that is 180°opposite to the side where bonding is initiated.
 18. The method of claim14, wherein the bonding is initiated about the center of thesubstrate(s) and the heating is applied to the entire periphery of thesubstrate(s).
 19. The method of claim 14, wherein heating is conductedby thermal conduction via transmission of heat from a support.
 20. Themethod of claim 19, wherein the support applies infrared radiation at awavelength of between 0.8 μm and 5 μm.
 21. The method of claim 14,wherein the heating is conducted to provide the bonding front with aspeed of above 0.6 and below 2 cm/s.
 22. The method of claim 21, whereinthe substrates are in the form of wafers of 300 mm in diameter, and theheating is conducted such that bonding is effected in an overall bondingtime of between 18 and 35 seconds to avoid forming edge voids.